In order to meet the demand for wireless data traffic, which has been increasing since the commercialization of a fourth-generation (4G) communication system, efforts are being made to develop an improved fifth-generation (5G) communication system or pre-5G communication system. For this reason, a 5G communication system or pre-5G communication system is referred to as a beyond 4G network communication system or a post Long Term Evolution (LTE) system.
To achieve a high data transmission rate, implementing a 5G communication system in an extremely high frequency (mmWave) band (for example, a 60 GHz band) is considered. To relieve the path loss of signals and to increase the transmission distance of signals in an extremely high frequency band, beamforming, massive Multiple-Input And Multiple-Output (massive MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna techniques are under discussion for a 5G communication system.
Further, to improve the network of the system, technical development in an evolved small cell, an advanced small cell, a cloud Radio Access Network (cloud RAN), an ultra-dense network, Device to Device (D2D) communication, wireless backhaul, a moving network, cooperative communication, Coordinated Multi-Points (COMP), and interference cancellation is progressing for the 5G communication system.
In addition, an Advanced Coding Modulation (ACM) scheme including Hybrid Frequency Shift Keying and Quadrature Amplitude Modulation (FQAM) and Sliding Window Superposition Coding (SWSC) as well as an advanced access technique including Filter Bank Multi Carrier (FBMC), Non Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA) are developing in the 5G system.
As the amount of mobile traffic has explosively increased with the introduction of smartphones, communication services are provided using an unlicensed band that is not licensed for a specific mobile network carrier, such as a Wireless Local Area Network (WLAN) or Bluetooth. A WLAN established with low costs uses an unlicensed band and thus is viewed as a major solution for cellular companies that have difficulty in dealing with traffic only with a cellular network.
For reference, a “licensed band” refers to a frequency resource licensed for a specific mobile network carrier, which is a frequency resource allocated for a particular purpose, that is, exclusively for LTE. Meanwhile, an “unlicensed band” refers to a communication frequency band that is not licensed for a specific mobile network carrier and is a shared band reserved for the general public. An unlicensed band may be used representatively for industrial, scientific, and medical communications. A WLAN (Wi-Fi) and Bluetooth are representative communication services using unlicensed bands. An unlicensed band may be available for any device that satisfies prescribed regulations and conditions for the use of the unlicensed band.
That is, when it is difficult to accommodate subscriber traffic in an area with a large floating population only with cellular base stations, most cellular network carriers induce subscribers to be provided with communication services through a WLAN Access Point (AP) by directly establishing a WLAN or in partnership with existing WLAN operators.
Meanwhile, a WLAN installed by a mobile network carrier to relieve traffic is a network that is basically different in feature from a cellular network. Therefore, the WLAN is insufficient to provide telecommunications subscribers with mobility or security, as if installing a cellular base station. Thus, a method is being studied that enables mobile network carriers to provide subscribers with mobile communication services by directly using a cellular radio technology in an unlicensed band that is not licensed to the mobile network carriers, instead of using an initial method of installing a WLAN AP, and discussions on Long Term Evolution-Unlicensed (LTE-U) are being conducted based on this concept.
An LTE-U system is a system in which a user equipment (UE) and a base station use carriers or channels existing in both a licensed band and an unlicensed band.
FIG. 1 is a view illustrating the concept of a UE using both a licensed band and an unlicensed band in LTE-U.
Referring to FIG. 1, the cellular UE 101 may use both the licensed band 103 and the unlicensed band 105. Further, a carrier in the licensed band 103 and a carrier in the unlicensed carrier 105 may coexist 107 using a carrier aggregation (CA) method 107.